1. Field of the Invention
The invention relates to mass spectrometry. In particular, this invention provides method and apparatus for reducing background noise caused by neutral metastable entities in a mass spectrometer. More particularly, instrument components are described for trapping secondary ions generated by bombardment of components by metastable entities.
2. Background Information
Mass spectrometry is an analytical technique that exploits the dependence of an ion trajectory through electric and magnetic fields on the ion mass/charge ratio. Typically the prevalence of component ions is measured as a function of mass/charge ratio and the data are assembled to generate a mass spectrum of a physical sample. The mass spectrum is useful, for example, for identifying compounds of unknown identity, determining the isotopic composition of elements in a known compound, resolving the structure of a compound and, with the use of calibrated standards, quantitating a compound in a sample.
Analysis by mass spectrometry entails a sequence of three component processes, each of which can be performed by any one of several types of devices. First, an ion source converts the sample into constituent ions. Second, after leaving the ion source, the charged species in the fragmented sample undergo sorting according to mass/charge ratio in a mass analyzer. Finally, the sorted ions enter a detector chamber, in which a detector converts each separated ion fraction into a signal indicative of its relative abundance. The attributes of the particular ion source, mass analyzer, and detector assembled to constitute a mass spectrometer tailor the capabilities of the instrument to analysis of particular sample types or to acquisition of specialized data.
For some applications, analysis by mass spectrometry can be enhanced by combination with other analytical techniques that separate the sample into constituents before ionization in the mass spectrograph. For example, in a common enhancement a gas chromatograph separates the sample into constituent components before it meets the spectrometer ion source, to improve distinction between compounds of relatively low molecular weight. This arrangement, termed gas chromatography-mass spectrometry (“GC/MS”), is widely used to identify unknown samples, especially in environmental analysis and drug, fire and explosives investigations.
The separative powers of gas chromatography enable GC/MS to identify substances to a much greater certainty than is possible using a mass spectrometry assembly alone. However, its necessary use of an inert carrier gas also introduces analytical difficulties in the form of background noise.
Some atoms of an inert carrier gas such as helium are excited to higher-energy metastable states in the mass spectrometer due, for example, to electron impact in the ion source or by collision with helium ions accelerated by the focusing elements. The common helium metastable states, e.g., 23S1, have energy levels of approximately 20 eV and can persist for several seconds.
The metastable atoms are uncharged and thus not focused by any of the ion optics. They tend to follow a line-of-sight path and bombard instrument components in their paths. The collisions generate secondary ions by a process known as Penning ionization, whereby ionization occurs due to a transfer of potential energy between atoms in an excited metastable state and a source of secondary ions. The secondary ion sources are believed primarily to be contaminants (for example, hydrocarbons)—arising from the pump oil, sample residue, and the reduced pressure atmosphere—on component surfaces.
Secondary ions created early in the matter stream, such as in the ion source or in the upstream portion of the analyzer, have the opportunity to be sorted by the analyzer and counted by the detector as representative of their chemical composition and structure. However, if the secondary ions are instead created near the exit from the analyzer, such as by striking the ion-focusing lens gating the detector chamber, or in the detector chamber itself, the secondary ions are not resolvable by the analyzer. If these late-created secondary ions enter the detector, they do so randomly, generating background noise. Metastable helium atoms are a major source of noise in GC/MS systems that use helium carrier gas.
Secondary ions can also be generated by excited neutral particles of other elements introduced, for example, by an inductively coupled plasma (“ICP”) ion source or by liquid chromatography-mass spectrometry (“LC/MS”) and other approaches that ionize the sample at atmospheric or reduced pressure.